Magnesium hydroxides crystal structure

Chromia Chromite Cr2Fe04 is the most commonly used chromium-containing mineral for ceramic formulations. This mineral has a spinel crystal structure, where the iron may be replaced by magnesium and aluminum. Chromite is used in ceramics largely as a refractory in the form of burned and chemically bonded bricks. For this purpose, a low-silica material is desired. When low silica is desired, chromic oxide is extracted from chromite by dissolution in add, removal of the iron impiu-ity by liquid—liquid extraction, and precipitation of the hydroxide, which is subsequently calcined to the oxide. Chromic oxide is used as a color additive to azes and enamels and in ferrite production to give magnetic materials. 

The decomposition of magnesium hydroxide has been evaluated as a process of nucleation and growth of MgO crystals within the brucite matrix. The formation of a defect layer of hydroxide structure, which suddenly recrystallizes to the cubic MgO structure when the fracture stress is exceeded in the defect layer, has been postulated (Freund et al., 1975 Guilliat et al., 1970 Garn et al., 1978 Lpnvik, 1978). The MgO crystals formed have an expanded cubic lattice that upon increasing calcination temperature gradually decreases in size until the equilibrium unit cell dimension is reached. 

The thermal decomposition of magnesium hydroxide under carefully controlled conditions, followed by subsequent rehydration, has been studied by means of nitrogen and neopentane adsorption isotherm measurements. Up to about 85% decomposition a uniform layered particle structure, consisting of plate-like microcrystallites intercalated by slit-shaped pores of width ca. 0.93 nm, gradually spreads from the outside towards the centre of each crystal. At this stage each particle consists of MgO of normal... 

Fluoride and hydroxide ions are saturated by bondB of total strength 1. This is achieved by two aluminum octahedral bonds, as in hydrar-gillite (Al(OH)t), with the structure shown in Figure 13-17, topaz (AljSiChF ), jmnyite, described below, and many other crystals, and also by three magnesium octahedra in brucite, Mg(OH)t, and other crystals. 

---